The art of surveying, or range finding, involves the determination of unknown positions, surfaces or volumes of objects using measurements of angles and distances. The determined angles and distances from a measuring instrument to points under survey may be used to calculate the coordinates of surveyed points relatively the measuring instrument. In order to make these measurements, an optical surveying instrument or geodetic instrument frequently comprises an electronic distance measuring (EDM) device which may be integrated in a so-called total station, see FIG. 1. A distance measuring total station combines electronic and optical components and is furthermore in general provided with a computer or control unit with writable information for controlling the measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system. Such a total station may comprise a telescope, which for example may be arranged with cross-hairs for sighting a target. Angle of rotation of the telescope, as well as angle of inclination of the telescope, may be measured relative to the target. Total stations comprising a camera are also known.
In conventional EDM, a light beam is emitted as a light pulse towards a target, and light reflected by the target is subsequently detected at the optical surveying instrument, such as a total station. Processing of the detected signal enables determination of distance to the target by means of, e.g., time-of-flight (TOF) or phase modulation techniques. Using the TOF technique, the time of flight of a light pulse that travels from the surveying instrument (the EDM device) to a target, is reflected at the target and returns to the surveying instrument (the EDM device) is measured, on the basis of which distance may be calculated. The power loss of the received signal determines the maximum possible range. Using a phase modulation technique, light of different frequencies is emitted from the surveying instrument to the target, whereby reflected light pulses are detected and the distance is calculated based on the phase difference between emitted and received pulses. As mentioned in the foregoing, once the angles and distances have been measured, the actual position of a surveyed target may be calculated.
In a conventional scanner, for example intended for use in industrial, surveying and/or construction applications, or in other applications, the light beam may be guided over a number of positions of interest at the surface of the target using a beam steering function. A light pulse is emitted towards each of the positions of interest and the light pulse that is reflected from each one of these positions is detected in order to determine the distance to each one of these positions. For example, using a LIDAR (Light Detection and Ranging) scanner, properties of scattered light may be measured to find range and/or other information of a distant target. In general, the distance to an object or surface is determined using laser pulses.
For increasing the measurement range in the TOF ranging applications, use of a master oscillator power amplifier (MOPA) may be advantageous since a high peak power can be achieved in the transmitted pulse, thus resulting in a longer range and higher measurement rate due to a higher signal-to-noise ratio. Higher output power is also advantageous in the phase modulation systems for the same reason. In a MOPA, a master laser is employed in combination with an optical amplifier used to amplify the output of the master laser. The master laser is often referred to as a seed laser. By using an optical amplifier to boost the output, the requirements on the seed laser may be mitigated, which allows reaching higher wavelength stability and spatial quality of the beam for the transmitter. A particular type of MOPA is realized with a microstructure semiconductor seed laser diode and an optically pumped fiber amplifier, which sometimes is referred to as a master oscillator fiber amplifier (MOFA).
To reach a high enough accuracy of distance measurements, short pulses should be used. Normally, optical pulses with duration τp of 1 to 50 ns are used, depending on application. Transmitters utilizing subnanosecond pulses are also known (cf., e.g., S. N. Vainshtein et. al., Rev. Sci. Instrum. vol. 71, no. 11, p. 4039-4044 (2000)). To provide optical pulses of duration of τp, the carrier life time in the laser τL should be about τp or shorter.
Let us assume that we want to obtain a 1 ns long optical pulse from the microstructure laser and that a laser with carrier lifetime τL<<1 ns is used. Using a microstructure laser diode as a seed laser requires an appropriate electrical laser driver in particular, a pulsed laser driver for TOE applications. Driving the seed laser with nanosecond or sub-nanosecond electrical pulses causes an intensive relaxation oscillation process, also called “spiking” in case relaxation oscillations are limited to one pulse, before lasing can be established. The origin of relaxation oscillations is directly related to the recombination processes in semiconductors (see, e.g., chapter 4 in “Handbook of semiconductor lasers and photonic integrated circuits”, Ed. Y. Suematsu and A. R. Adams, Chapman & Hall, 1994). In particular, characteristic time and amplitude of relaxation oscillations depend on spontaneous and stimulated emission relaxation times τs and τph, as well as on number of carriers and photons (see, e.g., p. 266 in “Handbook of semiconductor lasers and photonic integrated circuits”, Ed. Y. Suematsu and A. R. Adams, Chapman & Hall, 1994).
According to Einstein's quantum theory of light, there are two categories of light emission processes (also described in the book of Suematsu and Adams). The transition probability of the first kind of light emission process is proportional to the existing photon density. This is called the stimulated emission process. The transition probability of the second kind of light emission process is independent of the photon density and is called the spontaneous emission process. When applying a short current pulse to the semiconductor laser diode, a large number of carriers are injected in the active area. If the concentration of carriers is high enough, which is above the threshold level, population inversion is achieved, wherein stimulated emission commences, which in turn results in a growing number of photons, i.e. the lasing starts. However, the density of photons of the first kind, corresponding to the stimulated emission, is dose to zero in the beginning of the process, and growing of the number of photons is very slow. Because of that, the increasing concentration of carriers under the current pumping does not immediately result in an increase of the photon density and the concentration of carriers overshoots the level corresponding to the equilibrium lasing condition, under which the growth in the number of carriers is compensated by the radiative recombination process. After the photon density becomes large enough to intensify the optical recombination process, the number of carriers drops down to the equilibrium level and a spiking pulse is emitted from the seed laser. The spiking pulse is amplified by the fiber amplifier and therefore is present in the transmitted signal together with the main, intended optical pulse. The desired pulse shape is thereby distorted which may cause a decrease in the distance measurement accuracy.
While parameters of the main pulse are determined by the amplitude and duration of the driving pulse, both amplitude and start time of the spiking pulse are less predictable. In addition, the spiking pulse has a spectrum different from that of the main pulse, and the spiking pulse might further be spatially different from the main pulse, which may cause additional errors in the distance and angle measurements. Therefore, it is desired to eliminate—or at least mitigate—the spiking process.
The spiking, or relaxation oscillations, may be mitigated in different ways. First, the laser may be driven in continuous wave (CW) mode, while the optical power is varied by means of a modulator, for example an acousto-optic modulator. This approach suffers from limited extinction ratio at the output as well as high insertion loss to the modulator. Also, the maximum output power is limited.
Another, more general, approach is to develop laser diodes producing less relaxation oscillation by optimizing the laser structure. For instance, width and length of the optical resonator has large impact on wavelength and power stabilizing. Presently available semiconductor laser diodes have highly optimized structure, so that they may provide single-mode output in a wide range of driving current. That significantly reduces relaxation oscillations while driving the laser above the threshold, but spiking is still present if the driving current rises sharply from zero.
A third way to mitigate spiking is to continuously operate the seed laser above the lasing threshold, though not in the CW mode. For ranging applications, however, such a solution is typically not an option. First, most ranging instruments and devices are battery-driven, thus in general having a limited supply of energy, while driving the laser above the threshold adds significantly to the power consumption. Second, continuous driving of the laser above the threshold causes continuous illumination of the target between the pulses, though at lower intensity, which, in turn, may reduce contrast and decrease measurement accuracy. The third and most serious drawback of continuously operating the laser above the threshold applies to a seed laser in combination with an optically pumped fiber amplifier as discussed in the above—the continuous application of a bias current exceeding the lasing threshold generally results in poor efficiency.